In forming a water-repellent film on the surface of a substrate such as glass or the like, the technique is known to form an undercoat of silicon oxide or the like between the substrate and the water-repellent layer in order to improve the bonding strength of the substrate with the water-repellent film and prevent the diffusion of an alkali component where the substrate contains the alkali component, thereby enhancing the durability of the water-repellent film.
As techniques for forming a water-repellent film having such an undercoat there have been known two techniques, i.e., technique which comprises forming an undercoat of silicon oxide or the like on a substrate, and then forming a water-repellent film on the undercoat to produce a two-layer structure and technique which comprises applying a mixture of an undercoat composition and a water-repellent component to a substrate, thereby forming a single layer comprising of an undercoat layer and a water-repellent layer. The latter technique requires less producing steps and thus exhibits better productivity than the former. Examples of the latter technique include those disclosed in JP-B-63-24554 (The term "JP-B" as used herein means an "examined Japanese patent publication"), JP-A-61-215235 (The term "JP-A" as used herein means an "unexamined Japanese patent application"), JP-A-64-68477, JP-A-64-68477, JP-A-4-338137, and JP-A-8-239653. The feature of these techniques will be described below.
JP-B-63-24554 discloses a water-repellent surface treating agent obtained by modifying a silanol oligomer (eicosamer to tetracontamer) with a fluoroalkylsilane.
JP-A-61-215235 discloses a water-repellent, stain-resistant low reflectance glass obtained by a process which comprises applying to the surface of a substrate a solution obtained by allowing a fluoroalkyl group-containing silane compound and a silane coupling agent to undergo hydrolysis reaction in an alcoholic solvent in the presence of acetic acid and an organic tin compound to produce a co-condensate, and then heat curing the coating.
JP-A-64-68477 discloses a process for the production of a water-repellent steel plate which comprises applying an alcohol solution containing an alkoxide such as silicon oxide and a fluoroalkylsilane to the surface of a steel plate, and then heating the coated material.
JP-A-4-338137 discloses a water-repellent glass obtained by applying a mixture of a silicon alkoxide, a substituted silicon alkoxide obtained by substituting some of alkoxyl groups in silicon alkoxide by fluoroalkyl group, an alcohol, water, and an acid or base to the surface of a glass substrate, and then calcining the coated material.
JP-A-8-239653 discloses a water-repellent article obtained by treating an article with a composition comprising a mixture of a perfluoroalkyl alkylsilane and a completely hydrolyzable silane.
The methods disclosed in the above JP-B-63-24554, JP-A-61-215235 and JP-A-4-338137 comprise allowing a fluoroalkyl group-containing silane and a silicon alkoxide to undergo hydrolysis in an alcohol solution in the presence of a catalyst to produce a polycondensate and a co-condensate.
Since the above methods involve the bonding of polymers, the resulting water-repellent film is liable to have voids, making the denseness thereof low. In order to enhance the denseness of the water-repellent film, a calcining step is required, which results in increase of the production cost.
The method disclosed in the above JP-A-64-68477 is effected in the absence of catalyst and thus exhibits a poor reactivity. The resulting water-repellent film has a reduced denseness. In order to enhance the denseness of the water-repellent film, a calcining step is required, resulting in increase of the production cost.
In the technique disclosed in the above JP-A-8-239653, a coating composition comprising a chlorosilane and a non-aqueous solvent is preferably used. However, non-aqueous solvents are expensive. The coating composition was duplicated. As a result, it was found that since this coating composition reacts with moisture in the air to undergo gelation that renders the composition instable, it is necessary to maintain the ambient humidity at 10%RH or less, and preferably nearly 0%RH. Thus, it is difficult to control the environmental condition under which the coating composition.
This technique was duplicated. As a result, it was found that the resulting water-repellent film has minute unevenness on the surface thereof and thus exhibits a poor scuff resistance.
The techniques were duplicated. As a result, it was found that all the resulting water-repellent articles exhibit a poor water droplet rollability.